Yeah, 3D is the key here. Look on Twitter - there's a lot of love for Prometheus from people who've already seen it. Yes there are negative vibes as well, but overall the posts seem positive. One constant is that it's generally agreed the movie is the best thing visually since Avatar - and that alone will have people giving it a shot.
Madagascar as a threat? - not sure, the last movie was slated, it's a pretty well worn format AND under-promoted.
I sincerely hope Prometheus makes $300m+ , I'd like to see more, with another director, some even more braver choices, and a return to Xeno infested madness.